Discrete Slip Surfaces Research Articles

Middle Pleistocene fluid infiltration with 10–15 ka recurrence within the seismic cycle of the active Monte Morrone Fault System (central Apennines, Italy)

This study integrates field, geochronological and geochemical data to constrain fluid circulation in the damage and core zone of the seismogenic Monte Morrone Fault System (MMFS), central Apennines (Italy). Faulting along the MMFS evolved from a diffuse deformation at the damage zone towards progressive localisation of a narrower fault core and, finally, to (re)activation of discrete slip surfaces at shallower crustal conditions. Multiple generations of carbonate mineralisations, including veins and slickfibers, occur along the main fault surfaces. Carbonate mineralisations are locally fractured and incorporated in the surrounding cataclasites, documenting repetitive structurally-controlled fluid infiltration during transient episodes of permeability creation and destruction. Stable carbon and oxygen isotopes of the carbonate mineralisations document a dominant meteoric water source probably mixed with deeper circulating waters having longer residence time. Clumped-isotope yield formation temperatures of vein and slickenfibers in the range between 23 and 40 °C. U-Th dating of carbonate mineralisations yield Middle Pleistocene ages (from 268 to 189 ka BP), with a 10–15-ka cyclicity that we link to the coseismic rejuvenation of the structural permeability in the fault zone. We propose that fault-related mineralisations recorded the interactions among tectonic deformation and climate during the Quaternary. Our study is the first documentation of fault-controlled recurrence intervals in fluid infiltration in a seismically active fault of central Apennines.

The Role of Grain Size and Effective Normal Stress on Localization and the Frictional Stability of Simulated Quartz Gouge

AbstractFor earthquakes to occur on tectonic faults deformation localizes onto discrete slip surfaces within the gouge material of the fault core. Here, we investigate how localization is promoted in quartz gouge, a material known to exhibit unstable stick‐slip behavior, by assessing the role of grain size and effective normal stress () on microstructural evolution and frictional behavior. We observe stable sliding, slow‐slip instability, and stick‐slip with increasing displacement. We find that initially fine‐grained gouge and high promotes unstable stick‐slip behavior, whereas slow‐slip is more prevalent at low . Microstructural analyses of the sheared gouges show that stick‐slip behavior occurs after localization of deformation on to discrete Y‐shears, which undergo a significant cataclastic grain size reduction relative to the bulk of the gouge, a process enhanced by high . Our results show that localization and instability are promoted by high effective normal stress and small grain size.

Deformation bands in volcaniclastic rocks – Insights from the Shihtiping tuffs, Coastal Range of Taiwan

Deformation bands have been extensively studied in sandstones, whereas far less is known about bands occurring in porous volcaniclastic rocks. Here we investigate spectacular outcrop exposures of late Miocene tuffaceous rocks in the Coastal Range of Taiwan, which host several deformation band types: (i) disaggregation-dominated, layer-bound, sub-vertically-dipping pure compaction bands (PCB); (ii) cataclastic, layer-bound, reverse-sense compactional shear bands (RCSB); and (iii) non-layer-bound and intensely cataclastic transverse-sense compactional shear bands (TCSB). RCSBs and TCSBs host discrete slip surfaces on individual bands. The bands formed in an overall compressive stress regime related to convergence of the Eurasian and Philippine Sea plates. PCBs and RCSBs formed first, whereas progressive burial caused a shift to a transverse stress state and formation of TCSBs. The occurrence of cataclasis in RCSBs but not PCBs is inferred to be shear-driven, rather than confining-pressure-driven. Our findings suggest that cataclasis in deformation bands in volcaniclastic rocks is bimodal. Shear localization preferentially affects weak glass shards, causing intense comminution of volcanic glass. Feldspar, pyroxene and amphibole phenocrysts are comparatively less crushed, and cataclasis is strongly controlled by mineralogic cleavage planes. We conclude that increasing glass content reduces shear resistance, and that deformation bands in volcaniclastic rocks effectively exhibit a strain-weakening behavior.

Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault, Wollaston Forland, NE Greenland)

Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered.

Revisiting the Hubbert–Rubey pore pressure model for overthrust faulting: Inferences from bedding-parallel detachment surfaces within Middle Devonian gas shale, the Appalachian Basin, USA

Both bedding-parallel slickensides and cleavage duplexes are forms of mesoscopic-scale detachment faulting populating black (Marcellus and Geneseo/Burket) and intervening gray (Mahantango) shales of the Middle Devonian, a section known for abnormal pore pressure below the Appalachian Plateau. The abundance and the orientation of slickensides and cleavage duplexes in the more organic-rich black shale relative to gray shale suggests that maturation-related abnormal pore pressure facilitates detachment, a mesoscopic manifestation of the Hubbert–Rubey pore pressure model for overthrust faulting. The former are discrete slip surfaces whereas the latter consists of nested, anastomosing slip surfaces, either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes. Cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the Appalachian foreland, regardless of local limb dip. Cleavage duplexes are most common near the stratigraphic maximum flooding surface, the organic-rich section most prone to develop maturation-related pore pressure in the Middle Devonian gas shales. Bedding-parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale. In both black and gray shales, slickensides are more abundant on the limbs of folds, an indication of pore-pressure-related flexural-slip folding. On the macroscopic scale, the Pine Mountain Block of the Southern Appalachian Mountains was enabled by a basal detachment cutting along the Upper Devonian Chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure. The Pine Mountain block is a large-scale overthrust showing little evidence of collapse of the hinterland side, a credible example of a pore-pressure-aided overthrust fault block of the type envisioned by the Hubbert–Rubey model.

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the fault-related rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California. This work provides insight into the physical and chemical properties, structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. Exhumed outcrops within the SAF system comprised of serpentinite-bearing protolith are examined for comparison at San Simeon, Goat Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple juxtaposed lenses of sheared, foliated siltstone and shale with block-in-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by scaly clay fabric with multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg-rich clays and serpentine minerals (saponite ± palygorskite, and lizardite ± chrysotile). Whole-rock geochemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material, with a total organic content of >1 % locally in the fault-related rocks. The faults sampled in the field are composed of meters-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized Coast Range Ophiolite. X-ray diffraction of outcrop samples shows that the foliated clay gouge is composed primarily of saponite and serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures and deformation mechanisms interpreted from the outcrop sites are remarkably similar to those observed in the SAFOD core. Micro-scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites and are often attributed to aseismic deformation with episodic seismic slip. The mineralogy and whole-rock geochemistry results indicate that the fault zone experienced transient fluid–rock interactions with fluids of varying chemical composition, including evidence for highly reducing, hydrocarbon-bearing fluids.

Kinematic description of crystal plasticity in the finite kinematic framework: A micromechanical understanding of F=FeFp

The plastic component of the deformation gradient plays a central role in finite kinematic models of plasticity. However, its characterization has been the source of extended debates in the literature and many important issues still remain unresolved. Some examples are the micromechanical understanding of F=FeFp with multiple active slip systems, the uniqueness of the decomposition, or the characterization of the plastic deformation without reference to the so-called intermediate configuration. In this paper, we shed some light to these issues via a two-dimensional kinematic analysis of the plastic deformation induced by discrete slip surfaces and the corresponding dislocation structures. In particular, we supply definitions for the elastic and plastic components of the deformation gradient as a function of the active slip systems without any a priori assumption on the decomposition of the total deformation gradient. These definitions are explicitly and uniquely given from the microstructure and do not make use of any unrealizable intermediate configuration. The analysis starts from a semi-continuous mathematical description of the deformation at the microscale, where the displacements are considered continuous everywhere in the domain except at the discrete slip surfaces, over which there is a displacement jump. At this scale, where the microstructure is resolved, the deformation is uniquely characterized from purely kinematic considerations and the elastic and plastic components of the deformation gradient can be defined based on physical arguments. These quantities are then passed to the continuous limit via homogenization, i.e. by increasing the number of slip surfaces to infinity and reducing the lattice parameter to zero. This continuum limit is computed for several illustrative examples, where the well-known multiplicative decomposition of the total deformation gradient is recovered. Additionally, by similar arguments, an expression of the dislocation density tensor is obtained as the limit of discrete dislocation densities which are well characterized within the semi-continuous model.

Fluid flow numerical experiments of faulted porous carbonates, Northwest Sicily (Italy)

A methodology to assess the effects of structural heterogeneities below seismic resolution in porous carbonate grainstones on reservoir performance during production is developed by integrating structural analysis, power law distributions, up-scaling, and numerical techniques. The novelty of the methodology consists of accounting for the buffering effects on permeability caused by compactive and cataclastic deformation bands. By using this methodology, a 3D deterministic field analogue and a 3D Discrete Fracture Network (DFN) representations of the reservoir/aquifer were built first, and then single-phase, steady-state fluid flow numerical experiments in an equivalent porous medium framework were performed. The field analogue is located along the Northwestern coast of Sicily (Italy) where Lower Pleistocene porous carbonate grainstones are crosscut by a strike-slip fault system. This fault system is made up of two conjugate sets of strike-slip shear structures recognized as single compactive shear bands (CSB), zones of compactive shear bands (ZB) and well-developed faults (DF), with discrete slip surfaces and cataclastic material. The permeability of these structures is up to three orders of magnitude less than the surrounding porous carbonate rocks. The fluid flow numerical experiments have been performed on the two aforementioned reservoir/aquifer descriptions to assess the effect of Sub-Seismic Resolutions Faults (SSRF), such as those observed in the outcrops, on fluid flow during production from a well, injection production in Enhanced Oil Recovery (EOR), and up-scaling to large cell size for regional flow simulation. Comparison of draw-down modeling in the DFN and the deterministic models show that results are similar with the exception of wells located in areas of intense strain localization with ZB and DF. The use of the DFN model is therefore an acceptable representation of the heterogeneities induced by SSRF in a reservoir/aquifer. Results of numerical computations show that, in structurally complex areas, SSRF-related ZB and DF might represent a drilling risk because they can enhance draw-down during production and EOR activities.

Segment linkage process at the origin of slip surface roughness: Evidence from the Dixie Valley fault

The dynamics and geometry of slip along a fault govern the distribution of heterogeneities and, inturn, these heterogeneities organize slip. Some of these heterogeneities are morphological and are fossilized in the topography of the fault plane, i.e., its roughness. In the present study, our goal is to gain a better understanding of the processes involved during the creation and destruction of fault roughness. Our analysis is focused on the Dixie Valley (Nevada) normal fault, which has a particular outcrop that offers the opportunity to characterize the relationship between fault surface geometry and fault zone internal architecture. The fault morphology was measured in the field using a Light Detection And Ranging (LiDAR) apparatus. The data indicate that the fault surface topography is self-affine and characterized by two power law exponents, one parallel (H//=0.6) and one perpendicular (H⊥=0.8) to the slip direction. Accordingly, self-affinity implies that the fault surface appears smoother as the spatial scale increases. More precisely, this self-affine property indicates that the standard deviation of the roughness amplitude scales as L0.6 where L is the distance along the fault in the slip direction. We propose that fault roughness generation is dominated by damage processes that leave an imprint on fault geometry in the form of elongated lenses. Indeed, the fault zone displays a network of anastomosed discrete slip surfaces that bound bumpy lenses of damaged rock elongated in the direction of slip and that give the fault a rough topography. Such lenses are also observed in many other faults worldwide. Symmetry axis measurements of the lenses found in the Dixie Valley fault reveal that their geometry is scale-dependent. The maximum thickness T of the lenses scales with their length l, measured in the slip direction, as T∝l0.6. Based on previous experimental studies on the fault growth process, we suggest that the multi-scale aggregation of lenses explains why the standard deviation of the roughness amplitude of the fault surface scales as L0.6. We propose that elastic interactions related to the linkage of many discrete slip surfaces, controlling the generation of multi-scale bumpy lenses, are one of the physical processes at the origin of fault roughness.

Deformation analysis of tuffaceous sediments in the Volcanic Tableland near Bishop, California

Small-scale brittle faults and fractures that cut bedded tuffaceous sediments of variable textures and grain sizes were studied in a 110-m-long cutbank exposure of poorly consolidated sediments at the southern erosional boundary of the Volcanic Tableland, Owens Valley, California. This study was motivated by the need to evaluate potential length scales for lateral flow in nonwelded bedded tuffs and tuffaceous sediments at Yucca Mountain, Nevada—the site of a potential high-level radioactive waste repository. Small-displacement ( 20 cm). Vertical fractures are present throughout the exposure, but fracture frequency is generally highest in the vicinity of larger faults. Fault zones are characterized by grain-size reduction and discrete slip surfaces, the number of which increases with increasing displacement. A semiquantitative stress field interpretation, based on tectonic constraints and reconstruction of overburden thicknesses, yields a simple history of burial, deformation, and exhumation under continuous tectonic extension. We interpret the deformation to include shear (faults), hybrid (faults, nonvertical fractures), and tensile (vertical fractures) failure of the tuffaceous sediments under conditions of low overburden stress (<2.5 MPa). The intersecting network of faults and fractures is characterized by grain comminution, cementation, and fracture dilation. These features, in conjunction with stratigraphic layering, likely produce anisotropic permeability, where maximum permeability is parallel to fault and fracture strike.

Tectonic geomorphology of the Red Rock fault, insights into segmentation and landscape evolution of a developing range front normal fault

The Red Rock fault, an active normal fault in southwest Montana, is reevaluated for fault segmentation using a multiproxy approach. Field characterization of soils coupled with radiocarbon ages in offset hanging wall fans and terraces constrain the rupture history at a number of locations along strike. These data are coupled with morphometric analysis of dated alluvial fan deposits and adjacent 2nd and 3rd order footwall drainage basins to further constrain fault kinematics and segmentation. Morphometric analysis of the Big Sheep Creek catchment, which drains a large area of the footwall range, provides a control on the spatial extent of fault influence on footwall topography. Three fault segments are recognized by this study, compared with the two segments recognized previously. Data from offset alluvial surfaces were synthesized with hanging wall fan and footwall drainage basin data to demonstrate a southward increase in displacement along the Red Rock fault since at least the latest Pleistocene, at a maximum rate of 1.5 mm/yr. Red Rock ruptures were mainly confined to segments, suggesting a fault behaving as several discrete slip surfaces; however, simultaneous rupture of adjacent segments during one event indicates some fault zone coalescence. A youthful Red Rock fault is supported by the absence of tectonic influences on footwall topography and is consistent with an eastward progression of normal fault development along the northern arm of the Yellowstone hot spot.

Fault rock analysis of the northern part of the Chelungpu Fault and its relation to earthquake faulting of the 1999 Chi‐Chi earthquake, Taiwan

Abstract The 1999 Chi‐Chi earthquake in Taiwan (Mw = 7.6) produced a surface rupture along the north–south‐striking Chelungpu thrust fault with pure dip‐slip (east side up) and left lateral strike‐slip displacements. Near‐field strong‐motion data for the northern part of the fault illustrate a distinct lack of the high‐frequency seismic radiation associated with a large slip (10–15 m) and a rapid slip velocity (2–4 m/s), suggesting a smooth seismic slip associated with low dynamic frictional resistance on the fault. A drillhole was constructed at shallow depths in the possible fault zones of the northern part of the Chelungpu Fault, which may have slipped during the 1999 earthquake. One of the zones consists of a 20‐cm‐thick, unconsolidated fault breccia with a chaotic texture lacking both discrete slip surfaces (e.g. Riedel shears) and grain crushing. Other possible fault zones are marked by the narrow (less than a few centimeters) gouge zone in which clayey material intrudes into the damaged zone outside of the gouge zone. These characteristic fault rock textures suggest that the slip mechanisms at shallow levels during the earthquake involved either granular flow of initially unconsolidated material or slip localization under elevated pore pressure along the narrow clayey gouge zone. Because both mechanisms lead to low dynamic frictional resistance on the fault, the rapid seismic slip in the deep portions of the fault (i.e. the source region of strong‐motion radiation) could have been accommodated by frictionless slip on the shallow portions of the fault. The combination of strong‐motion data and fault rock analysis suggests that smooth slip associated with low dynamic friction occurred on both the deep and shallow portions of the fault, resulting in a large slip between the source region and the surface in the northern region.

Driving mechanisms for &gt;40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

In the NW North American Cordillera, the Cascades core region of the Coast Plutonic Complex underwent Late Cretaceous (&gt;96 Ma to locally 73 Ma) SW‐NE contraction and crustal thickening followed by dextral transpression (∼73 to 55 Ma), then transtension (&lt;55 Ma). Exhumation occurred during all three phases. During contraction, slow exhumation (∼0.6 mm/yr) occurred along the margins of the core, driven by isostatic rebound and erosion, and faster exhumation (&gt;3 mm/yr) by local thrusting in regions undergoing crustal thickening. In the central part of the core (Chelan block), &gt;40 km of exhumation occurred between 91 and 45 Ma, about half of which occurred during early contraction (driven by thrusting) and half during top‐to‐north, arc‐oblique shear during reactivation of a midcrustal Cretaceous thrust, the Dinkelman decollement. The footwall of this thrust consists of the Swakane Biotite Gneiss, a Cretaceous, metaclastic assemblage with recorded pressures of 10–12 kbar, no arc‐related magmatism, and structures dominated by pervasive top‐to‐north shearing. The hanging wall consists of the Napeequa Complex, an oceanic assemblage with recorded pressures of 6–12 kbar, voluminous arc‐related magmatism, and complex structures indicating early top‐to‐WSW shearing, younger top‐to‐north shearing, and widespread folding. In the Napeequa, top‐to‐north shearing started by 73 Ma during melt‐present conditions at pressures ≥6 kbar. Top‐to‐north shearing in both hanging wall and footwall continued during exhumation (∼1.6 mm/yr) and cooling to greenschist facies conditions during which slip became increasingly localized, eventually resulting in formation of pseudotachylite on discrete slip surfaces. We suggest that arc‐oblique extension was driven by along‐arc heterogeneity in displacements/erosion, initially during transpression and underplating of continental sediments, and later during transtension.

Information on Fault Orientation from Unoriented Cores

Unoriented core data can be integrated with other data types, such as seismic data, dipmeter data, and stratigraphic isochore or isopach data, to provide a unique solution for bedding orientation. In addition, such an integrated approach will help to identify erroneous data and interpretations. Once a unique solution for bedding orientation is obtained, this information can be used to find the orientation of faults and deformation bands as observed in core data. Although larger scale faults commonly are difficult to core due to brecciation and poor coherency of the fault rock, structural studies demonstrate that faults (with discrete slip surfaces) in sandstones are associated with numerous deformation bands in damage zones that are typically several meters wide. Within the fault zone itself, the deformation bands commonly are subparallel to the larger scale fault. Analyses of the orientation of deformation bands within the fault zone thus may yield information on the orientation of the larger scale fault. Based on such an integrated approach, it is possible to find information on faults on a subseismic scale that would otherwise be difficult or impossible to obtain. Quality control against seismically resolvable faults from the Gullfaks field, northern North Sea, demonstrates that this simple approach is valid and can provide important additional information for the structural interpreter. The results can be used to optimize well planning and enhance the understanding of fluid flow patterns in the reservoir.

Numerical simulations of granular shear zones using the distinct element method: 1. Shear zone kinematics and the micromechanics of localization

Two‐dimensional numerical simulations were conducted using the distinct element method (DEM) to examine the influences of particle size distribution (PSD) and interparticle friction μp on the nature of deformation in granular fault gouge. Particle fracture was not allowed in this implementation but points in PSD space were examined by constructing assemblages of particles with self‐similar size distributions defined by the two‐dimensional power law exponent D. For these numerical “experiments,” D ranged from 0.81 to 2.60, where D=1.60 represents the two‐dimensional equivalent of a characteristic PSD to which cataclastically deforming gouge is thought to evolve. Experiments presented here used μp values of 0.10 and 0.50 and were conducted using normal stress σn on the shear zone walls of 70 MPa. Shear strain within these simulated assemblages was accommodated by intermittent displacement along discrete slip surfaces, alternating between broadly distributed deformation along multiple slip planes and highly localized deformation along a single, sharply defined, subhorizontal zone of slip. Slip planes corresponded in orientation and sense of shear to shear structures observed in natural gouge zones, specifically Riedel and Y shears; the oblique Riedel shears showed more extreme orientations than typical, but their geometries were consistent with those predicted for low‐strength Coulomb materials. The character of deformation in the shear zone varied with PSD due to changes in the relative importance of interparticle slip and rolling as deformation mechanisms. A high degree of frictional coupling between large rolling particles in low D (coarse‐grained) assemblages resulted in wide zones of slip and broadly distributed deformation. In higher D assemblages (D &gt;= 1.60), small rolling particles self‐organized into columns that separated large rolling particles, causing a reduction in frictional resistance within the deforming assemblage. This unusual particle configuration appears to depend on a critical abundance of small particles achieved at D ≈ 1.60 and may enable strain localization in both real and simulated granular assemblages.

Décollement processes at the Nankai accretionary margin, southeast Japan: Propagation, deformation, and dewatering

The décollement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the décollement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this décollement zone may be unique from that of most thrust faults. We propose that the décollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai décollement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the décollement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of “cementation” in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protodécollement subsequently accommodates shear displacements along discrete fractures.

Cathodoluminescence Petrography of Microstructural Fabrics in Deformed Limestones: ABSTRACT

Cathodoluminescence petrography has been shown to be a valuable tool for analyzing strain microfabrics in deformed rocks. This study shows that cathodoluminescence petrography is useful for illustrating the details of the varied and heterogeneous nature of deformation mechanisms involved in the formation of deformation twins in calcite crystals. The advantage of viewing grains and syntaxial calcite cements in some deformed skeletal limestones is that finely banded luminescent growth zones in the cements commonly act as very sensitive displacement markers. The detailed structure associated with deformation bands can be observed, and the sense of displacement clearly determined. Deformation microfabrics apparent from displacement of the luminescent growth zones in calcite cements commonly are coincident with deformation twin lamellae that are visible in transmitted light. Cathodoluminescence petrography shows that modes of deformation associated with the twin lamellae vary between ductile deformation (formation of chevron-like microfolds in the luminescent zones) to offset of luminescent zones along discrete slip surfaces, to combinations of both mechanisms. Additionally, apparent ductile kink geometries in deformation twins observed in transmitted light are shown to result from twin development in microfracture-filling cements, when samples are viewed under cathodoluminescence. this study further illustrates the usefulness of cathodoluminescence petrography for studies ofmore » microstructural fabrics in deformed limestones.« less